1. Field
Methods and apparatuses consistent with exemplary embodiments relate to measurement technology, and more particularly, to measuring a geometric variable with respect to a nanostructure by means of scattered light investigation while scanning an object having a nanostructure.
2. Description of the Related Art
In micro-lithography technology developed for manufacturing semiconductor chips, there is a recent trend to reduce a critical dimension (CD) of a manufactured structure. CD signifies a size of a nanostructure to be examined and has a value of about tens of nanometers. Currently, a CD that may be embodied is about 30 nm and will be 20 nm soon.
For mass production of a semiconductor structure having a very small CD, a demand for accuracy and reliability of measurement equipment, speed, and economical efficiency of a measurement process have been increased. Also, a conventional measurement method using a scanning electron microscope (SEM) and an atomic force microscope (AFM) is very slow and expensive particularly when examining a chip having a well-known topology in which critical dimensions of structures are slightly different from required values. For the measurement of the type above, optical methods based on technologies such as ellipsometry [see reference 1] and scatterometry [see reference 2] have been developed. In particular, a well-known optical critical dimension (OCD) method [see reference 3] is used to identify a semiconductor structure having a CD smaller than a Rayleigh limit.
These optical analysis methods have their merits and demerits. In the OCD method, dependence of sub-wavelength structure reflectivity from critical dimension value, wavelength of incident irradiation, and angle of incidence are used. There are two types of methods: one is to fix a wavelength of incident light and measure a degree of dependence on reflectivity with respect to an incident angle of light irradiated onto an object by a through-angle scanning method and the other is to fix an incident angle and measure a degree of dependence on reflectivity with respect to a wavelength by a through-wavelength scanning method. In the second method, spectrums of irradiation of incident and reflected light are measured and a degree of dependence on reflectivity with respect to a wavelength is calculated based on the spectrum of the incident and reflected light. The measured dependence is compared with a dependence calculated with respect to various CD values and the best coincidence of measured curve and calculated curve gives a required CD value.
Although the OCD technology is widely used in a semiconductor manufacturing process, it is impossible to perform an analysis of a non-periodic structure, a low periodic structure, and a structure formed of one or more isolated objects by the technology.
The non-periodic and isolated objects may be analyzed by a through-focus scanning optical microscopy (TSOM) [see reference 4] based on analysis of a non-contrast defocused image of an object recorded using an optical microscope while scanning the object in an optical axis direction.
In the TSOM technology, a mechanical scanning system for moving an object through-focus at an accuracy of tens of nanometers is a basic one and simultaneously is the weakest in view of reliability of a module for measuring a TSOM-plant. As the size of an object decreases and scanning steps decreases, a demand for reduction of scanning steps and the accuracy of positioning of an object increases. In this situation, when vibrations exist, the accuracy and reliability in measurement are generally lowered.